The present invention relates to an apparatus and method for calibrating gamma cameras and more particularly to an apparatus and method for calibrating gamma cameras using an electronic source.
Gamma cameras are used in medical and other devices, typically, to detect the intensity and the spatial position of radioactive energy. In medical devices, the gamma camera detects radioactive energy from radioactive materials that have been placed by medical personnel in, for example, the blood stream of a patient for performing medical tests and imaging of various vital organs.
The gamma camera typically includes a gamma camera detector having a scintillation crystal that is positioned above an array of photo-multipliers. The scintillation crystal absorbs the radioactive energy from the radioactive materials. The absorbed radioactive energy is converted into energy signals that can be detected by the array of photo-multipliers. The detected energy signal is converted into an electrical signal by the array of photo-multipliers. Typically, the array of photomultipliers is connected to a resistor matrix that provides a ratio of displacement signals and intensity signals based on the intensity and spatial position of the radioactive material in relation to the array of photo-multipliers. The resistor matrix is connected to various electronics that interpret the ratio of displacement signals and the intensity signals to provide a video display and/or image of the medial test being performed.
From time to time, the gamma camera is calibrated to ensure the readings measured and data collected by the gamma camera are accurate. The calibration typically adjusts the spatial gain and offset of the array of photo-multipliers and other electronics to account for any environmentally caused electronic drift. Currently, skilled technicians perform the calibration by using several radioactive sources having varying radioactive intensities, such as, for example, americium (Am), cobalt (Co) and technetium (Tc). During the calibration, the technician places the radioactive sources on the scintillation crystal at various predefined locations that correspond to specific displacement signal to intensity signal ratios. At these predefined locations, the technician measures the signal ratios, and the gamma camera is adjusted such that the measured signal ratios are accurate.
The use of radioactive sources during the calibration presents many problems. For example, the storage and transportation of the radioactive sources are regulated by governmental agencies and strict compliance with the regulations enforced by these governmental agencies is required. Compliance with the governmental regulations increases the cost of the calibration. Typically, in order to perform the calibration of the gamma camera, a technician must travel to the facility where the gamma camera is physically located. As such, the technician must transport the radioactive source to perform the calibration. During transit, the technician must comply with the governmental regulations that concern storing and transporting the radioactive sources. Since compliance with the regulation is expensive, the cost of the calibration is increased simply because the technician must transport and store the radioactive sources for calibration. Therefore, a method and apparatus for calibrating the gamma cameras is desired that eliminates or reduces the use of radioactive sources and, thus, eliminates and/or reduces the costs associated with the storage and transportation of the radioactive sources.
In addition, since the cost of calibrating the gamma camera is high because the radioactive sources are necessary, the gamma cameras are calibrated less frequently. Infrequent calibration presents many problems, such as inaccurate readings and incorrect operation of the device. As such, it is desired that the calibration of the gamma camera be performed frequently. Therefore, an apparatus and method for calibrating gamma cameras is desired such that the gamma camera can be frequently calibrated at a reasonable cost.
In one exemplary embodiment, an electronic source used during calibration of a gamma camera is provided. In this embodiment, the gamma camera includes a scintillation crystal that is positioned above a photo-multiplier. A preamplifier is connected to the photo-multiplier, and the pre-amplifier includes an electrical output. The electronic source used during calibration of the gamma camera comprises a current source. A predetermined current signal is produced by the current source, and the predetermined current signal is representative of an intensity produced by the photo-multiplier when subjected to gamma radiation from a predetermined radioactive event that is positioned proximate to the scintillation crystal. An intensity selection switch is connected to the current source, and the intensity selection switch is adjustable to a conductive position. A reference current setting is also connected to the intensity selection switch. When the intensity selection switch is adjusted to the conductive position, the reference current setting is supplied to the current source and instructs the current source to produce the predetermined current signal.
A position selector switch is also connected to the current source to receive the predetermined current signal. The position selector switch comprises a plurality of connective positions, and a first of the plurality of connective positions supplies the predetermined current signal to a first output of the electronic source. A first potentiometer is connected to a second of the plurality of connective positions and produces a first ratio of current signals from the predetermined current signal. The first ratio of current signals is supplied to a second output of the electronic source. A second potentiometer is connected to a third of the plurality of connective positions and produces a second ratio of current signals from the predetermined current signal. The second ratio of current signals is supplied to a third output of the electronic source. The first output, the second output and the third output are connected to replace the electrical output of the pre-amplifier during calibration of the gamma camera. In addition, the first output, the second output and the third output, respectively, supply the predetermined current signal, the first ratio of current signals and the second ratio of current signals, respectively, in lieu of energy signals produced by the gamma camera in response to a radioactive source during calibration.
An exemplary method of calibrating a gamma camera comprises determining the intensity produced by the photo-multiplier when subjected to gamma radiation from a radioactive event proximate to the scintillation crystal. An x-axis displacement signal is determined that corresponds to an x-axis position of the radioactive event proximate to the scintillation crystal. In addition, a y-axis displacement signal is determined that corresponds to a y-axis position of the radioactive event proximate to the scintillation crystal.
The electronic source is connected to replace the electrical output of the pre-amplifier. The electronic source is set to a center energy level that is substantially equal to the intensity produced by the photo-multiplier when subjected to gamma radiation from the radioactive event. In addition, the electronic source is set to an x-axis energy level that is substantially equal to the x-axis displacement signal. Also, the electronic source is set to a y-axis energy level that is substantially equal to the y-axis displacement signal. A calibration of the gamma camera is performed using the center energy level, the x-axis displacement signal and the y-axis displacement signal from the electronic source in lieu of energy signals produced by the gamma camera in response to a radioactive source during calibration.
During the calibration of the gamma camera, the center energy level, the x-axis energy level and the y-axis energy level can be scaled to predetermined energy levels that correspond to an intensity produced by the photo-multiplier in response to a predetermined radioactive event. In one embodiment, scaling the outputs of the current source via adjustment of the reference current settings of the intensity selector also scales the energy levels at the outputs of the electronic source.